The present invention relates to an apparatus for controlling the flow rate of a fluid through a line and, more particularly, for limiting the maximum volumetric flow rate of a fluid through a line.
There are many flow control valves which limit the volumetric flow rate of a fluid through a line. Such valves are commonly provided with a moveable control element which is urged in one direction by the fluid pressure drop across the element, and in the opposite direction by a pre-compressed (i.e., preloaded) spring. The position of the control element effects the size of a control area through which the fluid flows and, hence, effects the pressure drop across the area and the fluid flow rate. As the pressure drop acting on the control element increases beyond a threshold pressure drop necessary to overcome the preload force of the spring, the spring deflects and the control element is accordingly displaced. As the control element is displaced, the control area decreases and the fluid pressure drop increases until the force on the element created by the pressure drop is equivalent to that of the spring force.
A drawback to the foregoing configuration is that the size of the control area is directly proportional to the displacement of the control element from an initial upstream position. The fluid passing through this control area, however, experiences a pressure drop inversely proportional to the square of the size of the control area. Consequently, the relationship between the pressure drop force and the displacement of the element is quadratic. In contrast, the relationship between the spring force and the displacement of the element is linear, rather than quadratic. As a result, the pressure drop force and the spring force are balanced at only one position of the element; at all other positions of the element there will be an imbalance. Hence, the control element will operate in only two positions, i.e., in a fully upstream, or open, position, and in a fully downstream, or closed, position.
Therefore, what is needed is a flow control limiter in which a maximum volumetric fluid flow rate is maintained when the fluid pressure drop exceeds the threshold pressure drop.